Concept of Friction
The Concept of Friction
Explain the concept for friction
Friction is the force which opposes (resists) motion of the body for example: If a block of wood is placed on a table it remains at rest Unless it is acted upon by an external force.
The applied force must exceed the maximum friction force between the block of wood and the table if the block is to be moved along the table.
Friction force between two surface in contact exists only when there is relative motion between the two contacting surface.
The
friction between two surfaces exists because of the nature of the
surface of the bodies in contact. (Strictly speaking all surfaces are
rough because they are made up of peaks and valleys)
friction between two surfaces exists because of the nature of the
surface of the bodies in contact. (Strictly speaking all surfaces are
rough because they are made up of peaks and valleys)
Demonstration
The
peak and valleys on the surfaces of bodies may be due to a random
arrangement of coarse particles, for example the surface of a gridding
stones.
peak and valleys on the surfaces of bodies may be due to a random
arrangement of coarse particles, for example the surface of a gridding
stones.
Smooth surfaces exert almost no friction. What happens when two objects are pushed?
When
one body is pushed against another, the peaks of one surface have to
rise up over the peaks of the other surface. Hence opposition to motion
or friction occurs the force acting to restore the bodies to their
original position.
one body is pushed against another, the peaks of one surface have to
rise up over the peaks of the other surface. Hence opposition to motion
or friction occurs the force acting to restore the bodies to their
original position.
The Advantages and Disadvantages of Friction in Daily Life
Realize the advantages and disadvantages of friction in daily life
Advantages of Friction
Thefriction force has several advantages which includes the following:
- It helps in walking process (because friction force stop us from slipping over).
- It
helps cars to move onroads easily due to friction between car tyres and
road (makesa car stay on road) NOTE: Most car tyres have deep dreads to
increase friction between the tyres and the road surface. - it
helps in car braking system. Brakes rely on friction between the brake
drum or pads and the wheels. In a bicycle, there are brake pads which
clamp onto the wheel to slow it down.
Disadvantages of friction
Frictional force has several dis-advantages which includes the following:
- Cause machinery to heat up and can cause wear and tear.
- Cause machinery to be less efficient.
- Cause machinery to produce noise.
Methods of Reducing Friction
Describe methods of reducing friction
Thedifferent methods ofreducing friction between surfaces includes the following:
- Polishing:Polishing the surface, reduces the irregularities and hence, reduces friction.
- Lubrication:It provides a layer of the lubricant on which the bodies can easily slide.
- Use
of ball-bearing or roller bearing:Roller bearing help to convert
sliding friction into a milder form-rolling friction. Rolling friction
is lesser than sliding friction. - Streamlining:Fast cars, boats,
planes etc., have a streamlined body. This is to allow air (or water in
the case of boats) to easily flow by, without offering much resistance.
Flying birds have streamlined bodies. - Use of correct combination
of surfaces in contact:Use of alloys on moving and sliding parts
reduces friction because alloys have a low coefficient of friction.
Roller
- It is a simple cylinder on which a body to be pulled RESTS.
- It is used to eliminates sliding friction.
- Conveyor belts use metal rollers.
Wheels
- These
arerollers that are fixed to a moving body, held in place by cylinders
or axles which are threaded in their centers. Example; a trailer towed
by vehicle. - Wheels are used to eliminate sliding friction.
- They
are used to reduce friction (the friction between surfaces can be
reduced by smoothing and polishing the surfaces in contact). - In
order to make surfaces slippery, a lubricant such as oil or graphite is
used. Oil and Grease are commonly used invehicles and machines to reduce
friction between moving parts. In engines, differential air is also an
effective lubricant in machine.
Laws of Friction
State laws of friction
The five laws of friction
- When an object is moving, the friction is proportional and perpendicular to the normal force (N).
- Friction is independent of the area of contact.
- The coefficient of static friction is slightly greater than the coefficient of kinetic friction.
- Within rather large limits, kinetic friction is independent of velocity.
- Friction depends upon the nature of the surfaces in contact.
The Coefficient of Friction
Determine the coefficient of friction
Coefficient of friction is the ratio of the frictional force that acts between two objectsin contact to the normal reaction, R.
Types of Coefficient of friction
There are three main types of coefficient of frictions which includes the following:
Coefficient of static friction is the ratio of the static friction to the normal reaction.
Us =Frs/R
Coefficient of dynamic friction US
Is the ratio of the dynamic friction to the normal reaction that acts onthe body.
Ud = Frd/R
Laws of Friction in Solving Problems
Apply laws of friction in solving problems
Demonstration to determine coefficient of dynamic friction.
Method of calculation
The coefficient of dynamic friction U is
U = Frictional force, Fr/Normal Reaction, R
U= Fr/R_______________________________ ( I)
By Resolving forces we get
Fr = W SinQ __________________________(II)
R= W CosQ ________________________(III)
Put Eqn (iii) ad (ii) into eqn (i)
U =W SinQ/Wc osQ
U =W SinQ/WCosQ
But SinQ/CosQ=TanQ
U=TanQ
Thus
TanQ = (AB/CB) FROM Angle ABC
But
U = TanQ
u = (ab/cb)
Example 2
Find
the static friction between a block of wood of mass 10kg and the table
on which it rests. A minimum force of 50N is required to make the block
just move on the table top.
the static friction between a block of wood of mass 10kg and the table
on which it rests. A minimum force of 50N is required to make the block
just move on the table top.
Solution
Limiting Friction, Fr = 50N
Normal Reaction, r =(10 X 10 ) = 100N
Coefficient of static friction; Us
Us = (Fr/ R)
Us = (50/100)
Us = 0.5
Therefore,coefficient of static friction, Us = 0.5
Example 3
A
mass is placed on an Inclined plane such that it moves at a constant
speed when tapped tightly. If the angle the plane makes with the
horizontal is 30º. Find the coefficient of dynamic friction.
mass is placed on an Inclined plane such that it moves at a constant
speed when tapped tightly. If the angle the plane makes with the
horizontal is 30º. Find the coefficient of dynamic friction.
The Coefficient of friction U =Fr/R
Fr = WSinQ/R = WCosQ
At Equilibrium
U =WSinQ/wCosQ
U = (W/W) (SinQ/CosQ)
But (SinQ/CosQ) = TanQ
U = Tan 30º
U = 0.56
Coefficient of friction, U = 0.56
